This application claims priority to an application entitled xe2x80x9cLow-Noise Amplifier for a Mobile Communication Terminalxe2x80x9d filed in the Korean Industrial Property Office on Dec. 29, 1999 and assigned Ser. No. 99-65239, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a mobile communication terminal, and in particular, to a low-noise amplifier for amplifying a received signal in a mobile communication terminal.
2. Description of the Related Art
FIG. 1 is a block diagram illustrating a receiver for a conventional mobile communication terminal.
Referring to FIG. 1, upon receipt of a radio signal, an RSSI (Received Signal Strength Indicator) detector 310 detects field strength of the received signal and provides information on the detected field strength to an automatic gain control (AGC) amplifier controller 320. The AGC amplifier controller 320 compares the detected field strength information provided from the RSSI detector 310 with a predetermined field strength value. If the detected field strength is lower than or equal to the predetermined field strength, the AGC amplifier controller 320 provides a power-up command to an AGC amplifier 360 and a low-noise amplifier (LNA) controller 330 in order to increase a power level of the received signal.
The low-noise amplifier controller 330 then provides a first bias circuit 150 with a preset voltage in order to enable a low-noise amplifier 100. The first bias circuit 150 generates a bias voltage according to the preset voltage provided from the low-noise amplifier controller 330 and provides the generated bias voltage to a base of a bipolar junction transistor (BJT) 140. Then, the BJT 140 is turned ON in response to the bias voltage and a current flowing through the BJT 140 is amplified and matched through an input matching circuit 120 and an output matching circuit 130. The output of the output matching circuit 130 is provided to a down-converter 340. The down-converter 340 mixes a local oscillation frequency provided from a local oscillator 350 with the received signal amplified by the low-noise amplifier 100. AGC amplifier 360 amplifies the signal output from the down-converter 340 to a predetermined level and outputs the amplified signal in an intermediate frequency (IF).
As shown in FIG. 1, the receiver with the Si BJT has increased current consumption due to the turn-on voltage. In particular, when the Si BJT is used for the low-noise amplifier, the noise factor characteristic is degraded. Further, a gain of the Si BJT is significantly variable depending on the current and the Si BJT consumes a comparatively large current to maintain its linearity, as compared with an GaAs MESFET (Metal Semiconductor Field-Effect Transistor). Particularly, in a mobile communication system using a high frequency (e.g., at around 2 GHz) such as PCS (Personal Communication System) or IMT-2000 system, the receiver generally uses a 2-stage low-noise amplifier. When using a plurality of low-noise amplifiers, the receiver has greatly increased current consumption due to an increase in number of Si BJTs used.
FIG. 2 is a detailed circuit diagram illustrating a modified low-noise amplifier for the receiver in the conventional mobile communication terminal.
Referring to FIG. 2, a received signal is applied to the input matching circuit 120 and the signal matched by the input matching circuit 120 is applied to a gate of a MESFET 230, the gate being connected to a self-bias resistor 200. A source bias resistor 220 is set to a specific resistance to maintain linearity. A capacitor 260 connected to the source of the MESFET 230 and a capacitor 240 connected to the drain of the MESFET 230 are used as a bypass capacitor. A drain of the MESFET 230 is connected to the power supply voltage Vdd through an inductor 250. A signal at the drain of the MESFET 230 is matched by the output matching circuit 130. However, the receiver with the MESFET, generally, must provide a negative voltage to the gate of the MESFET as a bias voltage, and a separate negative voltage generator is therefore needed to provide the negative voltage. Therefore, as shown in FIG. 2, a positive voltage being provided to the source through the source bias resistor 220 on a self-bias basis is used as a gate bias voltage. However, when the gate voltage is controlled by using the source bias resistor 220, there is a limitation in controlling the current because the source bias resistor 220 has a fixed resistance.
It is, therefore, an object of the present invention to provide a low-noise amplifier with minimized current consumption.
It is another object of the present invention to provide a low-noise amplifier which can maintain linearity with the least current consumption.
To achieve the above and other objects, there is provided a low-noise amplifier for a mobile communication terminal. In the low-noise amplifier, a low-noise amplifier controller receives a signal-to-noise ratio (SNR) of a received signal, and outputs a control signal of a preset voltage when the SNR is lower than a predetermined SNR. A bias circuit increases a bias current according to the control signal output from the low-noise amplifier controller. A low-noise amplifier amplifies the received signal to a preset level according to the bias current provided from the bias circuit.